The efficiency of gas turbine engines depends significantly on the operating temperature of the various engine components with increased operating temperatures resulting in increased efficiencies. The search for increased efficiencies has led to and continues to result in the development of heat-resistant nickel-base superalloys which can withstand increasingly high temperatures yet maintain their basic material properties.
The casting processes used with early generations of iron, cobalt and nickel-base superalloys, commonly referred to as conventionally cast superalloys, generally produced gas turbine engine components whose microstructures consisted of a multitude of equiaxed grains of random crystallographic orientation with grain boundaries between the grains.
Improvements in the ability of conventional superalloys to withstand higher temperatures without impairing other needed qualities, such as strength and oxidation resistance, was achieved through alloy development and the introduction of improved processing techniques. Those improvements followed from findings that the strength of such superalloys, and other important characteristics, were dependent upon the strengths of the grain boundaries. To strengthen such conventional superalloys, initial efforts were aimed at strengthening the grain boundaries by the addition of various grain boundary strengthening elements such as carbon (C), boron (B), zironcium (Zr), and hafnium (Hf) and by the removal of deleterious impurities such as lead (Pb) or bismuth (Bi) which tended to segregate at and weaken the grain boundaries.
Efforts to further increase strength levels in conventional nickel-base superalloys by preferentially orienting the grain boundaries parallel to the growth, i.e., solidification direction, were subsequently initiated. Preferential orientation of the grains generally results in a grain structure of long, slender columnar grains oriented substantially parallel to a single crystallographic direction and minimizes or eliminates grain boundaries transverse to the growth direction. The process used, directional solidification (DS), is described, for instance, in U.S. Pat. No. 3,897,815 - Smashey. The disclosures of all the U.S. Pat. Nos. referred to herein are hereby incorporated herein by reference.
Compared with conventionally cast superalloy articles, directionally solidified (DS'd) articles exhibited increased strength when the columnar grains were aligned parallel to both the solidification direction and the principal stress axis due to the elimination or minimization of grain boundaries transverse to the principal stress axis. In addition, DS provided an increase in other properties, such as ductility and resistance to low cycle fatigue, due to the preferred grain orientation. However, reduced strength and ductility properties still existed in the transverse directions due to the presence of longitudinal columnar grain boundaries in such DS'd articles. Additions of Hf, C, B, and Zr were utilized to improve the transverse grain boundary strength of such alloys as was done previously in conventional equiaxed nickel-base superalloys. However, large additions of these elements acted as melting point depressants and resulted in limitations in heat treatment which did not allow the development of desired microstructures for maximum strengths within such directionally solidified superalloys.
It had been recognized for some time that articles could be cast in various shapes as a single crystal, thus eliminating grain boundaries altogether. A logical step then was to modify the DS process to enable solidification of superalloy articles as single crystals to elimiate longitudinally extending high angle grain boundaries previously found in DS'd articles.
There then began continuing efforts in the development of processes for the casting of single crystal articles, such as blades and vanes, useful in gas turbine engines and for superalloys particularly tailored to such casting processes and such applications. Examples of such process and alloy development are found, for example, in U.S. Pat. Nos. 3,494,709-Piearcey; 3,915,761-Tschinkel et al.; 4,116,723-Gell et al.; 4,209,348-Duhl et al.; 4,453,588-Goulette et al.; and 4,459,160-Meetham et al.
Another type of alloy having high strength at elevated temperatures, combined with good resistance to hot corrosion and oxidation, and, therefore, useful as articles, particularly blades and vanes, for gas turbine engines are the eutectic alloys. Suitable eutectic alloys may be cast using a directional solidification method, sometimes referred to as planar front solidification, so as to produce columnar grained, polycrystalline, anisotropic composite articles having a superalloy matrix with reinforcing fibers embedded therein which are aligned substantially parallel to the solidification direction.
Illustratively, in the nickel-base eutectic superalloy described in U.S. Pat. No. 4,305,761-Bruch et al., aligned eutectic reinforcing metallic carbide fibers are embedded in a gamma-gamma prime matrix in which the gamma prime strengthening phase is dispersed in the gamma phase. The reinforcing metallic carbide (MC or monocarbide) fibers are those of which the metal preferably is principally Ta, but can include, in addition, such metals as Mo, W, V, Re and Cb as may be included in the alloy. Other nickel-base eutectic superalloys are described, for example, in U.S. Pat. Nos. 4,284,430-Henry and 4,292,076-Gigliotti et al.
In the case of the nickel-aluminum-molybdenum-tantalum alloy described in U.S. Pat. No. 4,288,259-Pearson et al., the structure comprises a gamma prime [Ni.sub.3 (Al,Ta)]matrix and an alpha body centered cubic (Mo) second phase in fibrous form. In U.S. Pat. No. 3,985,582-Bibring et al., there are described eutectic superalloy articles wherein carbide fibers are embedded in a columnar grained, polycrystalline matrix of a complex refractory superalloy and wherein the matrix also contains a fine precipitates of carbide of the same nature as the fibers.
One significant impediment to the use of eutectic superalloys in gas turbine engine applications is their high cost associated with the near equilibrium, generally very slow, growth rates, typically on the order of 1/4 inch/hour, required.